1. Field of the Invention
This invention relates to a method and apparatus for controlling an atmosphere in a heat treatment furnace, and more particularly relates to a method and apparatus for controlling an atmosphere in a heat treatment furnace for carrying out a gas carburizing, carbonitriding or bright controlled atmosphere heat treatment, etc.
2. Description of the Prior Art
In conventional heat treatment methods, such as gas carburizing of metals, a mixture of a hydrocarbon gas and air is converted into an endothermic gas through the use of an endothermic type converted gas generator. The endothermic gas is introduced into the furnace, and an enriched hydrocarbon gas is added to the furnace in order to obtain a predetermined carbon potential.
However, recently, in order to enhance the quality, and to reduce the treatment time and running cost, gas generators are no longer desirable. Rather, introducing a hydrocarbon series gas and an oxidizing gas are directly into the furnace to carry out the carburizing in the furnace has been proposed.
Such a method is described in Japanese Patent Applications Laid-Open Nos. 54931/1979, 159567/1986 and 63260/1992.
However, the carburizing speed in the direct carburizing method is varied on a large scale according to the carburizing time and the diffusion time. In the carburizing time, the main effect is the direct decomposition of the hydrocarbon gas, etc. (raw gas) and in the diffusion time, the main effect is the Boundouard reaction.
In the carburizing time, the degree of the decomposition varies due to the quantity of the hydrocarbon gas to be introduced directly into the furnace and the temperature of the atmosphere in the furnace as well as the type of goods to be treated in the furnace. As a result, hydrocarbon gas in excess of the amount required for carburizing is piled as soot in the furnace, thereby potentially subjecting the goods to be treated to soot.
If the heat treatment is carried out in the sooting range, the service life of the oxygen sensor becomes reduced.